a) Field of the Invention
This invention relates to a method and system for the control of a clutch for a fluid coupling, which clutch is suited for use in controlling a torque converter and a clutch arranged in association with an automatic transmission for a vehicle.
b) Description of the Related Art
An automatic transmission for an automotive vehicle generally includes a shift mechanism constructed of planetary gears, so that sun gears, planetary gears and the like are selectively driven or made standstill by hydraulic friction engagement elements, such as a hydraulic wet multi-plate clutch and hydraulic band brakes, to obtain a desired speed range. Further, a torque converter as a fluid coupling is interposed between an engine and the shift mechanism, whereby the torque converter transmits an engine torque at an increased rate to the shift mechanism upon starting or the like or absorbs a shock upon shifting, sudden acceleration or deceleration, or the like.
An automatic transmission, however, has a drawback that the fuel consumption is poorer compared with a manual transmission because the automatic transmission is operated with a slip in a torque converter. To overcome this drawback, many of recent automatic transmissions are provided with a lockup clutch (hereinafter referred to as a "damper clutch") in a torque converter so that in a predetermined drive range, an input shaft (namely, an output shaft on a side of an engine) and an output shaft (namely, an input shaft on a side of a shift mechanism) of the torque convertor are connected directly. Operation modes of the damper clutch include, in addition to a non-slip direct connection mode in which the input shaft and the output shaft are completely connected together during power-on running at relatively high revolutions, a deceleration-time slip mode in which the input shaft and the output shaft are connected together with a small degree of slipping therebetween during decelerated running, as well as a slip mode in which the input shaft and the output shaft are connected together during power-on running at relatively low revolutions while allowing the input shaft and the output shaft to slip as many as about several tens rotations per minute.
A sufficient pressure (i.e., a high hydraulic pressure) is,applied to avoid slipping of the damper clutch in the non-slip direct connection mode, whereas in each of the deceleration-time slip mode and the slip mode, an apply (i.e., engaging) pressure which has been suitably regulated (to a low hydraulic pressure), through a damper clutch control valve (hereinafter simply referred to as the "control valve"), to achieve a target slip amount is applied to the damper clutch.
In general, a spool valve is employed as the control valve and the spool valve is operated by a control hydraulic pressure, duty-controlled by a solenoid valve. When changing from a non-slip direct connection range or a non-direct connection range to a deceleration-time slip range or a slip range, the solenoid valve is temporarily driven at a predetermined drive duty ratio and is then feedback controlled to achieve a target slip amount. Upon this feedback control, the hydraulic pressure is gradually corrected from a preset feedback control initiating hydraulic pressure to obtain a desired connecting hydraulic pressure.
The term "duty ratio" as used herein means a drive duty ratio of a solenoid valve. This duty ratio indicates the percentage of a time during which the drive valve is activated. For example, "70% duty ratio" means that the solenoid valve is activated for only 0.7 second out of 1 second. Similarly, "10% duty ratio" means that the solenoid valve is activated for only 0.1 second out of 1 second.
In the case of a valve which is closed while a solenoid valve is not being activated but is opened upon activation of the solenoid valve, the drive duty ratio of the solenoid valve indicates the percentage of a time, during which the valve is opened, per unit time. In the case of a valve which remains open while a solenoid valve is not being activated but is closed upon activation of the solenoid valve, the drive duty ratio of the solenoid valve indicates the percentage of a time, during which the valve is closed, per unit time.
In the embodiment of the present invention to be described subsequently herein, the drive duty ratio of a solenoid valve indicates the percentage of a time, during which a valve for feeding an apply pressure, per unit time so that the duty ratio corresponds to the apply pressure.
The above-mentioned feedback control initiating hydraulic pressure has heretofore been determined based on a relationship between the drive duty ratio of the solenoid valve and an apply pressure as ascertained through an experiment or the like.
In automatic transmissions, however, the drive duty ratio for obtaining a desired pressure to be applied generally varies to a considerable extent from one transmission to another for differences in the shift mechanism, control valves and the like among individual transmissions and also manufacturing errors in solenoid valves. In addition, a pressure required to achieve a direct connection may by itself vary due to wearing or the like of individual members in the course of use over a long period of time.
When a solenoid valve whose drive duty ratio has been shifted to a lower side is used, for example, a hydraulic pressure higher than that needed is outputted if the solenoid valve is driven at an ordinary feedback control initiating duty ratio. This results in a sudden engagement of the clutch, thereby involving the potential problem that a shock may occur. It is therefore necessary to set the feedback control initiating hydraulic pressure at a sufficiently low value.
When a solenoid valve whose drive duty ratio has been shifted to a higher side is used, on the other hand, it takes a relatively long time until the deceleration-time slip mode or the slip mode is actually achieved even if feedback control is initiated. This makes it difficult to effectively reduce fuel consumption.